One of nine essentialamino acids required in the human
diet. As with any amino acid, lysine is a building block for making human proteins. Lysine helps with calcium absorption, the formation of collagen, and bone growth.

Lysine is found in most protein food sources but there's a lot more of it in animal protein (fish, meat, dairy products) than other sources.
Grains and cereal generally lack lysine (with the exception of wheat germ, buckwheat, amaranth, quinoa, and rice). Companies like Novartis are working to "improve" upon nature by developing genetically modified versions of corn that contains lysine.

1Parts of the AAA pathway of lysine degradation are also found in animals, contributing to the theory that the Animal and Fungal kingdoms are much more more closely related than Plants and Fungi, as was once assumed.